The Problem: Flutter's Service Worker vs CDN Architecture

Understanding Flutter's Service Worker Implementation

Flutter's service worker uses a content-hash based caching strategy:

Asset Manifest with MD5 Hashes: The service worker contains a RESOURCES object that maps file paths to their MD5 hashes
Hash-Based Change Detection: When a new build is deployed, the service worker compares hashes to determine which assets need updating
Cache-First Strategy: The service worker intercepts all requests and serves from its local cache, only fetching from network when not cached
No Query Parameters: Flutter does NOT append hash-based query parameters to URLs - it uses the same URLs and relies on the service worker update lifecycle

Here's the critical point: Flutter's service worker does not use these MD5 hashes for cache busting via query parameters. Instead, it:

Fetches resources using their original URLs (e.g., /main.dart.js)
Relies on the service worker update mechanism to detect when the RESOURCES manifest has changed
Assumes that fetching the same URL will return the new content when hashes differ

This approach works well when serving directly from a web server. However, it breaks down when you introduce a CDN.

The CDN <> Service Worker Cache Coherency Problem

Here's what happens when you use Flutter's service worker with a CDN like Cloudflare:

Initial State: You deploy main.dart.js with aggressive caching headers (Cache-Control: max-age=31536000)
CDN Caches: Cloudflare edge nodes cache this file based on the HTTP headers
New Deployment: You update your app, generating a new main.dart.js with a different MD5 hash
Service Worker Detects Change: The service worker sees the hash mismatch and requests the "new" file
CDN Serves Stale Content: Since the URL is the same (/main.dart.js), Cloudflare serves the cached version
Result: The service worker receives the old file despite knowing it needs the new one

So we have a couple of options to counter these issues:

Option 1: No-Cache Headers

Cache-Control: no-cache, must-revalidate

Defeats the purpose of having a CDN
Every request hits your origin server
Eliminates global edge caching benefits

Option 2: Cache Busting Query Parameters

If you need to implement cache busting anyway (e.g., main.dart.js?v=12345), you're essentially:

Duplicating the service worker's versioning logic
Adding complexity to your build process
Making the service worker almost redundant

The Incompatibility

The core issue is that Flutter's service worker and CDNs operate on different caching paradigms:

Service Worker: Content-based caching (MD5 hashes)
CDN: URL-based caching (same URL = same content)

These two approaches are incompatible without additional coordination. Why This Matters for Production Applications ?

Immediate Updates Are Impossible: Users continue seeing old versions until CDN caches expire
Hard Refresh Doesn't Help: Even Ctrl+Shift+R won't bypass CDN caches
CDN Effectiveness Is Compromised: Edge nodes cannot cache content aggressively without risking stale content delivery. You'd need to either use short TTLs (reducing cache hit rates) or manually purge caches on every deployment (operationally complex)

The Solution: Remove the Service Worker

Without a service worker:

Implement simple query-parameter cache busting: main.dart.js?v=timestamp
Configure aggressive CDN caching: Cache-Control: max-age=31536000, immutable
Let browsers and CDNs handle caching based on URLs
Updates are immediate on page refresh

This approach aligns with how CDNs are designed to work and eliminates the cache coherency problem entirely.

Flutter's Position

The Flutter team recognizes these limitations and is deprecating the default service worker. They acknowledge that the current implementation doesn't play well with modern web infrastructure.

How to correctly remove a service worker...

When you remove the service worker registration from your index.html, you might expect the service worker to disappear. It doesn't. Here's why:

Service workers persist after installation: Once installed in a user's browser, the service worker continues to run independently of your code
No registration ≠ no service worker: The service worker intercepts requests regardless of whether your current code registers it
Users stuck with old behavior: All existing users continue to experience the caching problems indefinitely

The high level steps to remove a service worker properly are...

Deploy a cleanup service worker at the same URL: Replace flutter_service_worker.js with cleanup code
Let the browser's update check do the work: Service workers automatically check for updates when users navigate to your site
The cleanup worker unregisters itself: When the browser fetches and installs the "updated" service worker, it runs the cleanup code

The Critical Discovery

Initially, we overcomplicated this by trying to:

Register a new cleanup service worker from JavaScript
Have it clean caches and then unregister itself
Remove the old service worker programmatically

This led to a problematic loop where the cleanup worker would register and unregister on every page load.

Finally came the dumb realization: You don't need to register the cleanup worker at all. The browser's standard service worker update mechanism will:

Check if flutter_service_worker.js has changed
Download and install the new version (our cleanup code)
Activate it, running our cleanup logic
The cleanup code then unregisters itself permanently

Here's the entire cleanup service worker:

// flutter_service_worker.js - Cleanup version
self.addEventListener('install', e => self.skipWaiting());

self.addEventListener('activate', e => {
    e.waitUntil((async () => {
        // Delete ALL caches
        for (const key of await caches.keys()) {
            await caches.delete(key);
        }
        // Take control of all clients
        await self.clients.claim();
        // Unregister ourselves
        await self.registration.unregister();
    })());
});

That's it. No fetch handler, no complex logic. Just:

Skip waiting to activate immediately
Delete all caches
Claim all clients to ensure cleanup affects current session
Unregister to remove the service worker completely

Implementing Cache Busting Without Service Workers

The Query Parameter Strategy

With the service worker removed, we need a different approach to ensure users get fresh content. The solution is straightforward: append version query parameters to all JavaScript files.

main.dart.js → main.dart.js?v=1754052153

This simple change enables aggressive CDN caching while guaranteeing fresh content on deployments.

Post-Build Cache Busting Script

After running flutter build web, we run a cache busting script that modifies the build output. Here is the script in essence:

#!/bin/bash
CACHE_BUST_VERSION=$(date +%s)

# Add cache busting to main.dart.js and other JS files
sed -i '' "s|main\.dart\.js\"|main.dart.js?v=${CACHE_BUST_VERSION}\"|g" build/web/index.html
sed -i '' "s|flutter\.js\"|flutter.js?v=${CACHE_BUST_VERSION}\"|g" build/web/index.html

# Handle deferred loading parts
find build/web -name "*.part.js" | while read part; do
  basename_part=$(basename "$part")
  sed -i '' "s|${basename_part}\"|${basename_part}?v=${CACHE_BUST_VERSION}\"|g" build/web/main.dart.js
done

Key points:

Uses timestamp (date +%s) for unique versions
Updates all JavaScript references in index.html
Handles deferred loading parts
Updates serviceWorkerVersion for backward compatibility during transition

Cloudflare Headers Configuration

With cache busting in place, we can configure aggressive caching headers. In your _headers file:

# Cache JavaScript files for 1 year
/*.js
  Cache-Control: public, max-age=31536000, immutable

# Never cache HTML
/*.html
  Cache-Control: no-cache, must-revalidate

# Cache assets with hashed filenames forever
/assets/*
  Cache-Control: public, max-age=31536000, immutable

# Cache WASM files for 1 year
/*.wasm
  Cache-Control: public, max-age=31536000, immutable

Why This Works

Unique URLs on Each Deploy: The timestamp ensures every deployment has unique URLs
CDN Can Cache Aggressively: One-year cache duration maximizes edge performance
Instant Updates: New URLs bypass all caches immediately
No Service Worker Complexity: Standard HTTP caching rules apply

Important Considerations

Cloudflare Caching Levels: Ensure your Cloudflare cache level is set to "Standard" (not "Ignore Query String") so query parameters are included in the cache key.

Build Process Integration: The cache busting script must run after flutter build web but before deployment to ensure all references are updated.

Deployment Simplicity: Every deployment gets a fresh timestamp, ensuring clean cache invalidation whether deploying new features or reverting to previous code.

Update: Evolution to Content-Based Cache Busting

After implementing the timestamp-based solution described above, we evolved our approach to something more sophisticated. We now use content-based cache busting with MD5 hashes—bringing back the efficiency of the original service worker without its CDN incompatibility.

Read our follow-up post: Content-Hashed Caching for Flutter Web to learn how we:

Implemented comprehensive URL patching inspired by Discord's Embedded App SDK
Created a content-based cache-busting system that only reloads changed files
Achieved the best of both worlds: efficient caching with CDN compatibility

Removing Flutter's Service Worker, how to do it correctly